Friday, March 02, 2012
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Near to the end of the story that starts with stars exploding in the Galaxy and ends with extra clouds gathering, a small but important paragraph was missing till now. From experiments in Copenhagen reported in 2006 and reconfirmed in 2011 in Aarhus and Geneva (CERN, CLOUD), cosmic rays coming from old supernovas can indeed make molecular clusters a few millionths of a millimetre wide, floating in the air. But can these aerosols really grow nearly a million times in mass to be large enough to become “cloud condensation nuclei” on which water droplets can form – as required by Henrik Svensmark’s cosmic theory of climate change?

Opponents pointed out that theoretical models said No, the growth of additional aerosols would be blocked by a resulting shortage of condensable gases like sulphuric acid in the atmosphere.

Not for the first time, an unexpected trick that Mother Nature had up her sleeve is revealed by experiment. The discovery is elegantly explained by a new way in which sulphuric acid forms in the atmosphere, as announced in a paper by Svensmark and two of his colleagues in Denmark’s National Space Institute in Copenhagen, Martin Enghoff and Jens Olaf Pepke Pedersen. They have submitted it to Physical Review Letters. A preprint is available on arXiv as 1202.5156:

A brief history. Last year’s attempts to dismiss the Aarhus and CERN results as inconsequential for climate change didn’t dismay the Danes. They knew there was something wrong with the current understanding because they had observational support for the whole chain from solar activity to cosmic rays to aerosols to clouds in the real atmosphere (Svensmark, Bondo and Svensmark 2009). In order to dig into the physics, they decided to rebuild, in the basement of the Space Institute, the 8 cubic metre experimental chamber SKYII which six years ago was used as the CLOUD prototype chamber at CERN,

In the limelight of the atmospheric drama, sulphuric acid is one of the commonest of trace gases and very important for both the formation and the growth of aerosols. When the Sun rises in the morning, its ultraviolet rays convert sulphur dioxide, ozone and water vapour in the air into sulphuric acid molecules. These are continuously lost as they club together with further water and a little ammonia into very small molecular clusters. Nevertheless, the concentration of sulphuric acid rises to a peak and then diminishes as the Sun goes down in the evening.

A clue that something more is going on comes from the persistence all through the night of sulphuric acid at about 10 per cent of the daytime maximum. If these molecules too are continuously lost, they must be replenished by a chemical reaction that doesn’t need ultraviolet light.

What did the new experiment called SKY2 show? Without going into technical details that you’ll find in the paper, let’s just say that the primary result flatly contradicts the theoretical prediction that the infant aerosols couldn’t grow up into cloud condensation nuclei. Here’s a figure from the paper.

Molecular clusters grow over time, in the SKY2 experiment in Copenhagen. The horizontal axis is scaled in nanometres (millionths of a millimetre) and each blue point shows the relative number of clusters of that size before and after the experimental runs. Anything over 1.0 means that growth has continued. In contrast, the red points illustrate a pessimistic prediction of previous theories, that growth should cease when the size passes 50 nanometres. On the other hand, the black curve running through the blue points shows what is to be expected if there is a continual supply of sulphuric acid. The persistent growth of clusters occurs only in the presence of gamma rays that simulate cosmic rays and set electrons free to influence the chemistry.

So what’s the explanation? What new pathway supplies the sulphuric acid needed to keep the growth going? The Danes recall a suggestion in their 2006 SKY report that electrons can act like catalysts, being used over and over again to promote chemical action. In the new paper they say:

A possible explanation could be that the charged clusters are producing additional [sulphuric acid] molecules from reactions involving negative ion chemistry of [ozone, sulphur dioxide and water], where a negative ion can be reused in a catalytic production of several [sulphuric acid molecules].

Depending on the concentrations of trace gases, several may mean dozens. And where do the electrons come from? They are liberated by cosmic rays raining down by night as well as by day. If the results of the experiment and these ideas are confirmed, there’s an amazing pay-off. The cosmic rays help to make the extra sulphuric acid that allows (1) a number of additional aerosols to form and (2) a larger number of aerosols to grow into cloud condensation nuclei. Without this second effect the aerosols would grow slowly and most of the extra aerosols would be lost before becoming large enough to seed clouds.

That ions liberated by cosmic rays promote a second pathway for forming sulphuric acid was already known from an experiment performed in Copenhagen in a collaboration with the University of Copenhagen and the Technical University of Tokyo (see the Enghoff et al. reference below). Depending on whether the sulphuric acid comes from ultraviolet action or is ion-induced, it has different signatures in the relative abundances of isotopes of sulphur. What’s more, the number of molecules made by the ion route greatly surpassed the number of ions available, again implying reuse of the electrons in a catalytic fashion.

To summarize the latest paper, the Svensmark, Enghoff and Pepke Pedersen abstract reads:

In experiments where ultraviolet light produces aerosols from trace amounts of ozone, sulphur dioxide, and water vapour, the number of additional small particles produced by ionization by gamma sources all grow up to diameters larger than 50 nm, appropriate for cloud condensation nuclei. This result contradicts both ion-free control experiments and also theoretical models that predict a decline in the response of larger particles due to an insufficiency of condensable gases (which leads to slower growth) and to larger losses by coagulation between the particles. This unpredicted experimental finding points to a process not included in current theoretical models, possibly an ion-induced formation of sulphuric acid in small clusters.

Scandals of a political character engulf climate physics these days, but future historians may shake their heads more sadly over scientific negligence. Isn’t it amazing that such a fundamental activity of sulphuric acid, going on over your head right now, has passed unnoticed since 1875 when cloud seeding was discovered, since 1996 when Svensmark found the link between cosmic rays and cloud cover, and since 2006 when the Danes suggested the catalytic role of electrons? Perhaps the experts were confused by the ever-present dislike of the role of the Sun in climate change.

So Svensmark and the small team in Copenhagen have had nearly all of the breakthroughs to themselves. And the chain of experimental and observational evidence is now much more secure:

Svensmark won’t comment publicly on the new paper until it’s accepted for publication. But I can report that, in conversation, he sounds like a man who has reached the end of a very long trek in defiance of continual opposition and mockery.

Henrik Svensmark, Jens Olaf Pepke Pedersen, Nigel Marsh, Martin Enghoff and Ulrik Uggerhøj, ‘Experimental Evidence for the Role of Ions in Particle Nucleation under Atmospheric Conditions’, Proceedings of the Royal Society A, Vol. 463, pp. 385–96, 2007 (online release 2006). This was the original SKY experiment in a basement in Copenhagen.

snail feedback (7)
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Luboš, our climate guru, maybe you could inform also about this research, if you are objective:

The world's oceans may be turning acidic faster today from human carbon emissions than they did during four major extinctions in the last 300 million years, when natural pulses of carbon sent global temperatures soaring, says a new study in Science. The study is the first of its kind to survey the geologic record for evidence of ocean acidification over this vast time period.

Dear Alexander, could you please kindly avoid similar brutally off-topic comments in the future?

This blog entry is about a serious topic of cosmoclimatology, not about the pH of the oceans.

The pH of the oceans may be changing at an "unprecedented" rate because no animal before homo sapiens was extracting and burning fossil fuels as quickly and cleverly as we are but the rate of pH increase is still negligible for all imaginable practical purposes, and whoever claims it's not is just a deluded crank.

If you want to talk about ocean acidification, I insist that you will start by learning the basic numbers on acidification and then you may be spamming the web while reflecting this basic knowledge. You have failed to do so now.

". . . cosmic rays coming from old supernovas can indeed make molecular clusters a few millionths of a millimetre wide, floating in the air. But can these aerosols really grow nearly a million times in mass to be large enough to become “cloud condensation nuclei”"

Lubos, I know you are just quoting here, but I'm a bit unclear as to how massive these cloud condensation nuclei have to be. Based on the above quote, we are starting with a "few millionths of a millimeter" and then growing nearly a million times. Are condensation nuclei really on the order of a millimeter wide? That seems pretty massive. Indeed, many water droplets are smaller than that.

Dear Eric, is the formatting unclear enough so you really missed that the article was written by Nigel Calder and not myself?

Cloud condensation nuclei have to be 50+ and possiblyi 200 nanometers in size. Only at this size, the droplets may naturally grow. One may estimate it by looking at the free energy gained by evaporation vs. by the surface tension.

I suppose that the "million times" growth referred to a factor multiplying the volume, so the linear dimension grew 100 times.

Thanks, Lubos. I saw the article the other day on another site, so knew it wasn't yours. Apologies if it sounded like I was implying you wrote it. My question was just related to the math. Incidentally, I think this is a notable result and bolster's Svensmark's work. Will be interesting to see how this shakes out in the coming years.

I think you're right they must be referring to volume, but even then, only roughly.

The article refers to a "few" nanometers as the starting point. Just to use nice round numbers, if we start with a 10 nanometer nuclei, and go from there all the way up to the 200 nanometer diameter, the increase in volume is only 8000x. (Sorry, formatting will probably be messed up.)

If we go all the way down to 2 nanometers for the starting point (which would seem to be less than the "few" nanometers referenced in the article, but anyway . . .), then we hit 1Mx exactly at the largest number you've listed for condensation nuclei (200 nanometers). I realize you've probably already done the math.

At any rate, I think you're right that they are referring to volume, which makes sense. Maybe the article is slightly imprecise in the numbers, but it's close enough that I can see where the numbers came from and they appear to be roughly lining up. I realize the point of the article wasn't to lay out precise quantifications but to report on the development, so I'm not nitpicking. Just taking a few minutes to make sure I'm on board with what they are saying, which I now am.

I liked this line: "Isn’t it amazing that such a fundamental activity of sulphuric acid, going on over your head right now, has passed unnoticed since 1875 . . . Perhaps the experts were confused by the ever-present dislike of the role of the Sun in climate change. So Svensmark and the small team in Copenhagen have had nearly all of the breakthroughs to themselves."

Back in August, Gavin Schmidt claimed that in order for the Svensmark theory to explain recent global warming, the following four points would need to be established:

"1. that increased nucleation gives rise to increased numbers of (much larger) cloud condensation nuclei (CCN)2. and that even in the presence of other CCN, ionisation changes can make a noticeable difference to total CCN3. and even if there were more CCN, you would need to show that this actually changed cloud properties significantly,4. and that given that change in cloud properties, you would need to show that it had a significant effect on radiative forcing."

Now 4 looks like something that is already well-established; maybe 3 is established too? Points 1 & 2 on the other hand look like more serious objections. They also claim an observation of decreasing trend in cosmic rays would be needed, although I recall reading at Nigel's blog about this too.

Am I right that this new paper claims to have demonstrated points 1 & 2? Sorry, I am a bit confused by the meaning of Gavin's points. Any general comments on how this experiment relates to RealClimate's comments?

I can't say anything too useful because to me, all the conditions you listed sound pretty much like tautologies. Clouds' existence depends on CCNs so whether or not and how many CCNs you have will surely make an O(1) impact on some properties of clouds.

The theory may have some holes but I don't think that any of the points you mentioned is a crisp articulation of the "real problem".